[SystemInfo API] Implement DisplayInfoProvider::UpdateDisplayUnitInfoForPlatform for Linux GTK

chrome/browser/extensions/api/system_display/display_info_provider_gtk.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/extensions/api/system_display/display_info_provider.h"
 
+#include <gdk/gdk.h>
+
+#include "ui/gfx/display.h"
+#include "ui/gfx/screen.h"
+
 namespace extensions {
 
 void DisplayInfoProvider::SetInfo(
     const std::string& display_id,
     const api::system_display::DisplayProperties& info,
     const SetInfoCallback& callback) {
   base::MessageLoopProxy::current()->PostTask(
       FROM_HERE,
       base::Bind(callback, false, "Not implemented"));
 }
 
 void DisplayInfoProvider::UpdateDisplayUnitInfoForPlatform(
     const gfx::Display& display,
     extensions::api::system_display::DisplayUnitInfo* unit) {
-  NOTIMPLEMENTED();
+  // TODO(Haojian): determine the DPI of the display
+  GdkScreen* screen = gdk_screen_get_default();
+  // The |id| in Display for GTK is the monitor index.
+  gint monitor_num = static_cast<gint>(display.id());
+  char* monitor_name = reinterpret_cast<char*>(gdk_screen_get_monitor_plug_name(
+       screen, monitor_num));
+  unit->name = std::string(monitor_name);
 }
 
 }  // namespace extensions